The invention relates generally to electric lamps formed with a pinched seal, in which a conductive foil is incorporated in the pinch. In particular, it relates to a molybdenum foil which is protected against oxidation by a layer which inhibits oxidation of the foil.
Electric lamps having a quartz glass lamp envelope frequently have outer current conductors of molybdenum which are connected with internal electrodes by a molybdenum foil. The foil is used in the area of the pinch seal. Being more flexible than the thicker molybdenum conductor, it is better able to absorb the stresses placed on the conductor in the pinch area. Molybdenum oxidizes rapidly in an oxidizing environment, such as air at temperatures of about 350° C. and higher. In the case of molybdenum foil used for hermetic pinch and vacuum-formed seals, this oxidation can result in an open circuit or can crack open the seal, either of which results in lamp failure. The oxidation reaction is thought to occur because during the sealing operation, microscopic passageways are formed around the lead wires as the vitreous material cools. These passageways permit oxygen to enter the foil area of the lamp seal.
Chromizing processes have been developed for reducing oxidation of an Mo—Nb pin-foil assembly during lamp operation. In such processes, a relatively thick layer of chromium is deposited on the foil. These processes often provide unsatisfactory results due to difficulties in process control. Additionally, the chromizing layer only allows moderate increases in the foil temperature before oxidation occurs. It has also been proposed to coat the molybdenum in the seal area which is exposed to oxidizing environments with an alkali metal silicate.